Diabetes is a lifelong metabolic disorder that affects 3.2 million people in the UK and over 347 million people worldwide. It is a leading cause of death with 1.5 million deaths annually. More than 80% of diabetes deaths occur in low- and middle-income countries. Approximately £10 billion is spent by the NHS on diabetes annually, which is almost 10 percent of the entire NHS budget.
Diabetes is a chronic disease in concomitant with severe secondary complications, mainly caused by poor glycaemia control. Actual treatment with exogenous insulin often fails to prevent and control these complications, leading to significant morbidity and mortality.
Diabetes mellitus is usually classified as type 1 or type 2 diabetes. The former results from failure and/or destruction of the insulin-producing (3-cells in the pancreas, often due to an autoimmune response. In contrast, type 2 is characterized by insulin resistance usually combined with an insulin secretory defect. Both forms of diabetes are characterised by hyperglycaemia which if not controlled adequately can lead to serious long-term complications including cardiovascular disease, stroke, kidney failure, foot ulcers and damage to the eyes. Current treatment of diabetes is aimed at lowering blood glucose levels with insulin the main stay of treatment for type I diabetes and combination of biguanides and sulphonylureas with or without insulin being used for type II diabetes. This treatment is highly demanding, invasive, expensive and not curative. In addition, relatively few patients truly achieve a normo-glycaemic state, though this is changing with the use of insulin infusion pumps, regulated by glucose sensing devices, where this is available and affordable.
Gene therapy offers the potential of a cure through continuous, endogenous production of insulin following a single therapeutic manoeuvre. Type I diabetes is well suited to somatic gene transfer approaches as it arises from a defect in a single protein, Insulin. Additionally, ectopic expression of functional human insulin has been demonstrated in a variety of different settings.
There have been many attempts at experimental gene and stem cell therapy to treat diabetes. However, the majority involve complicated protocols and ex-vivo cell manipulations, followed by transplantation of transfected or trans-differentiated cells to the diabetic animals. In vivo direct gene therapy is theoretically attractive by eliminating in vitro stages with the attendant risks of infection and failure of the gene transfected cell transplants to survive and develop an adequate blood supply.
The liver, with its embryological origins in primitive endoderm similar to the pancreas and the important role it plays in glucose homeostasis, has been considered as a suitable site to synthesize insulin. Amongst vectors available for gene therapy, there has been considerable interest in adeno-associated viral vectors (AAV) in part due to their excellent safety profile. These vectors, when pseudotyped with serotype 8 capsid (AAV8), have shown a remarkable tropism for the liver. Unlike other viral vectors, e.g. adenovirus and poxvirus, the prevalence of neutralizing antibodies to AAV8 in humans is low, enabling effective transduction of the liver following a simple systemic injection of AAV8, as illustrated in patients with severe haemophilia B (Nathwani, et al 2011). In 2011, Nathwani and colleagues reported that a single injection of AAV8-Factor IX in clinical treatment of patients with Hemophilia B had encouraging long-lasting therapeutic effects.
Gene therapy for diabetes could be facilitated by the development of a small expression cassette driven by a liver specific promoter driving the furin cleavable human proinsulin gene.
Insulin is synthesized as part of a 11.5 kDa precursor protein called pro-insulin. Pro-insulin is packaged into secretory vesicles where it is processed into mature insulin (6 kDa).
WO 2005/097981 discloses a herpes simplex viral vector which may contain a nucleotide sequence encoding insulin. However, this nucleotide sequence is the wild type sequence and does not provide particularly high levels of expression.